docs: add contributing guidelines for collaborators

CONTRIBUTING.md

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+# Express Collaborator Guide
+
+This document contains information for Collaborators of the express codemod regarding maintaining the code, documentation.
+
+## Getting Started
+
+The steps below will give you a general idea of how to prepare your local environment for the express codemods and general steps for getting things done and landing your contribution.
+
+1. [Create an issue](https://github.com/expressjs/codemod/issues/new) for the
+   bug you want to fix or the feature that you want to add.
+2. Create your own [fork](https://github.com/expressjs/codemod) on GitHub
+3. Clone your fork using SSH, GitHub CLI, or HTTPS.
+```sh
+git clone [email protected]:<YOUR_GITHUB_USERNAME>/codemod.git # SSH
+git clone https://github.com/<YOUR_GITHUB_USERNAME>/codemod.git # HTTPS
+gh repo clone <YOUR_GITHUB_USERNAME>/codemod # GitHub CLI
+```
+4. Change into the nodejs.org directory.
+```sh
+cd codemod
+```
+5. Create a remote to keep your fork and local clone up-to-date.
+```sh
+git remote add upstream [email protected]:expressjs/codemod.git # SSH
+git remote add upstream https://github.com/expressjs/codemod.git # HTTPS
+gh repo sync expressjs/codemod # GitHub CLI
+```
+6. Create a new branch for your work.
+```sh
+git checkout -b name-of-your-branch
+```
+7. Run the following to install the dependencies and start a local build of your work.
+```sh
+npm ci # installs this project's dependencies
+npm run dev # starts a development environment
+```
+8. Perform your changes
+9. Ensure your code is linted by running `npm run lint` -- fix any issue you
+   see listed.
+10. If the tests pass, you can commit your changes to your fork and then create
+   a pull request from there. Make sure to reference your issue from the pull
+   request comments by including the issue number e.g. `#123`.
+
+## How to add codemods
+
+We use `jscodeshift` to create and run the codemods. To add a new codemod for Express, we follow the following process.
+
+1. Create a new file in the `transforms` directory. For example, `transforms/pluralized-methods.ts`.
+
+2. Write your codemod. Here's an example that pluralizes Express methods:
+
+```typescript
+// filepath: codemod/transforms/pluralized-methods.ts
+import type { API, FileInfo } from 'jscodeshift'
+import { Identifier, identifier } from 'jscodeshift'
+import { getParsedFile } from '../utils/parse'
+
+export default function transformer(file: FileInfo, _api: API): string {
+  const parsedFile = getParsedFile(file)
+
+  const identifierNamesToReplace = ['acceptsLanguage', 'acceptsCharset', 'acceptsEncoding']
+
+  for (const singular of identifierNamesToReplace) {
+    const plural = `${singular}s`
+
+    parsedFile
+      .find(Identifier, {
+        name: singular,
+      })
+      .replaceWith(() => identifier(plural))
+  }
+
+  return parsedFile.toSource()
+}
+```
+
+3. Add tests to verify the functionality of the codemod
+    - A new file is created in the `/transforms/__test__` directory with the same name as the codemod with the following content
+    ```ts
+    // filepath: codemod/transforms/__test__/pluralized-methods.ts
+
+    import { testSpecBuilder } from './util'
+
+    testSpecBuilder('magic-redirect')
+    ```
+    - Two new files are created, `name-codemod.input.ts` and `name_codemod.output.ts`, inside the `/transforms/__testfixtures__` directory
+        - The files ending in `.input.ts ` contain the content that should be changed by the codemod
+        - The `.output.ts` files contain the content that should be present after the codemod has been correctly applied.
+
+4. To make the codemod visible within the CLI, the `config.ts` file is modified, where a brief description of the codemod, its name, and the version of Express to which the migration should be applied are added.
+
+## Developer's Certificate of Origin 1.1
+
+```text
+By making a contribution to this project, I certify that:
+
+ (a) The contribution was created in whole or in part by me and I
+     have the right to submit it under the open source license
+     indicated in the file; or
+
+ (b) The contribution is based upon previous work that, to the best
+     of my knowledge, is covered under an appropriate open source
+     license and I have the right under that license to submit that
+     work with modifications, whether created in whole or in part
+     by me, under the same open source license (unless I am
+     permitted to submit under a different license), as indicated
+     in the file; or
+
+ (c) The contribution was provided directly to me by some other
+     person who certified (a), (b) or (c) and I have not modified
+     it.
+
+ (d) I understand and agree that this project and the contribution
+     are public and that a record of the contribution (including all
+     personal information I submit with it, including my sign-off) is
+     maintained indefinitely and may be redistributed consistent with
+     this project or the open source license(s) involved.
+```